The government of the United States of America may have rights to this invention pursuant to NREL Subcontract No. ZM-0-19033-1 and EPRI Agreement No. RP3505-01.
This invention pertains to photovoltaic and electronic devices fabricated of amorphous silicon and its alloys, and more particularly to a plasma deposition process for enhancing the optical and electrical properties of photovoltaic and electronic devices.
Solar cells and other photovoltaic devices convert solar radiation and other light into usable electrical energy. The energy conversion occurs as the result of the photovoltaic effect. Solar radiation (sunlight) impinging on a photovoltaic device and absorbed by an active region of semi-conductor material, e.g. an intrinsic i-layer of amorphous silicon, generates electron-hole pairs in the active region. The electrons and holes are separated by an electric field of a junction in the photovoltaic device. The separation of the electrons and holes by the junction results in the generation of an electric current and voltage. The electrons flow toward the region of the semiconductor material having an n-type conductivity. The holes flow toward the region of the semiconductor material having a p-type conductivity. Current will flow through an external circuit connecting the n-type region to the p-type region as long as light continues to generate electron-hole pairs in the photovoltaic device.
An amorphous silicon solar cell is comprised of a body of hydrogenated amorphous silicon (a-Si:H) material, which can be formed in a glow discharge of silane. Such cells can be of the type described in U.S. Pat. No. 4,064,521 entitled "Semiconductor Device Having A Body Of Amorphous Silicon" which, issued to David E. Carlson on Dec. 20, 1977. Within the body of the cell there is an electric field which results from the different conductivity types of the semiconductor regions comprising the body.
Amorphous silicon solar cells are often fabricated by the glow discharge of silane (SiH.sub.4). The process of glow discharge involves the discharge of energy through a gas at relatively low pressure and high temperature in a partially evacuated chamber. A typical process for fabricating an amorphous silicon solar cell comprises placing a substrate on a heated element within a vacuum chamber. A screen electrode, or grid, is connected to one terminal of a power supply, and a second electrode is connected to the other terminal of the power supply such that the substrate is between the second electrode and the screen electrode. While silane, at low pressure, is admitted into the vacuum chamber, a glow discharge is established between the two electrodes and an amorphous silicon film deposits upon the substrate.
Amorphous silicon can be doped by adding impurities to the silane. For example, the first dopant may be diborane (B.sub.2 H.sub.6), which is added to the silane to form a p-type amorphous silicon layer. After the p-type layer has been formed to a thickness on the order of 100 Angstroms (.ANG.), the diborane flow is stopped to form an intrinsic region having a thickness on the order of a few 1000 Angstroms. Thereafter, an n-type dopant, such as phosphine (PH.sub.3), is added to the silane flow in order to form an n-type amorphous silicon layer having a thickness of a few 100 Angstroms. On the n-type layer, a transparent, conductive layer is formed. Usually zinc oxide (ZnO) is used to form the transparent conductive layer.
Single-junction amorphous silicon solar cells can be formed with a p-i-n structure or an n-i-p structure. The substrate of the solar cell can be made of glass or a metal, such as aluminum, niobium, titanium, chromium, iron, bismuth, antimony or steel. If a glass substrate is used, a transparent, conductive coating, such as tin oxide (SnO.sub.2) can be applied to the glass substrate prior to forming the amorphous silicon. A metallic contact can be formed on the back of the substrate.
Current output of a photovoltaic device is maximized by increasing the total number of photons of differing energy and wavelength which are absorbed by the semiconductor material. The solar spectrum roughly spans the region of wavelength from about 300 nanometers to about 2200 nanometers, which corresponds to from about 4.2 eV to about 0.59 eV, respectively. The portion of the solar spectrum which is absorbed by the photovoltaic device is determined by the size of the bandgap energy of the semiconductor material. Crystalline silicon (c-Si) has a bandgap energy of about 1.1 eV. Solar radiation (sunlight) having an energy less than the bandgap energy is not absorbed by the semiconductor material and, therefore, does not contribute to the generation of electricity (current and voltage) of the photovoltaic device.
Amorphous silicon solar cells were first constructed two decades ago. The first generation of solar cell devices suffered from low efficiency as well as from photodegradation, a phenomenon now known as the Staebler-Wronski effect. Over the years, there have been many improvements in both initial performance and stability, so that presently, conventional large area multi-junction modules composed of many monolithically connected cells exhibit stabilized performance below 9%.
The first cells were Schottky barrier devices and had an open circuit voltage (V.sub.oc) of about 0.55V. A series of improvements in the structure of solar cells, including first the use of p-i-n homojunctions and subsequently p-SiC/i,n-Si heterojunctions brought substantial improvements in the open circuit voltage (V.sub.oc). Further improvement resulted from the insertion of a thin i-SiC layer between the p-SiC layer and the i-layer. Increases in V.sub.oc can be realized by improvements of the transparent conductor, by use of improved p-layers by improvements in the p-i interface layer, by incorporating higher bandgap i-layers into the cell. The last two techniques, improving the p-i interface and widening the gap of the intrinsic layer by using SiC alloys or varying the i-layer deposition conditions can, however, result in material which has lower resistance to photodegradation. The exact details of the deposition process and device structure are very important.
The performance of hydrogenated amorphous silicon (a-Si:H)based solar cells degrades upon light exposure. Deterioration of electronic properties of the intrinsic a-Si:H layer in the p-i-n solar cell is believed to be responsible for most of the degradation in device performance. The a-Si:H i-layer has been traditionally deposited from a glow discharge of pure silane at a substrate temperature around 250.degree. C. For devices made under such conditions, the efficiency of the solar cell degrades linearly with the logarithm of light exposure time until approximately 10,000 hours of light soaking at 100 mW/cm.sup.2 intensity and then saturates beyond this time frame.
A characteristic of single-junction silicon cells made by conventional procedures (without hydrogen dilution) is its loss (degradation) over time of conversion efficiency, i.e. efficiency of the solar cell in converting sunlight to electricity. Efficiency plotted versus the log of time provided a straight line which was observed in some cases to extend to at least 10,000 hours. With few exceptions, the efficiency was observed to continue to decay as time went on with no deviation from the straight line dependence. The slope of the straight line was principally a function of the i-layer thickness. A good rule of thumb is that the loss of efficiency per decade of time, is the i-layer thickness in angstroms divided by 300. As an example, a 4000 .ANG. thick cell would lose 4000/300=13%/decade. It was found that proper design of the p-i interface layer could improve initial efficiency without affecting the slope of this line. However, if the interface layer design was not correct, the slope of the efficiency-log time curve would increase. Going to lower deposition temperatures without hydrogen dilution, would substantially increase the slope of the line. At low deposition rates, e.g. from 4-10 .ANG./sec, the degradation rate is unaffected by deposition rate. However, at high rates the degradation rate increases.
The degradation rate of a multi-junction cell is about the average of the rates of the component cells comprising it. As an example, a single-junction amorphous silicon cell with a 4000 .ANG. thick i-layer formed at high temperatures and no hydrogen dilution, might lose 40% of its initial efficiency in 1000 hours of light soaking. A Si/Si tandem having the same total thickness of i-layer (front i-layer 700 .ANG., back i-layer 3300 .ANG.) would lose only about 20% (front i-layer loss about 7%, back i-layer about 33%).
Stability is a characteristic of how a solar cell's efficiency (performance) will change or degrade under continuous or pulsed solar illumination. One of the biggest challenges in a-Si:H photovoltaic (PV) technology has been the instability problem. Conventional prior art hydrogenated amorphous silicon (a-Si:H) solar cells can degrade greatly with light soaking, particularly for cells with thick i-layers. It is well known in the photovoltaic industry that the addition of even a small amount of carbon to amorphous silicon (a-Si:H) will make the solar cell very unstable, even for thin cells with less than 1000 .ANG. i-layer thickness. Previously deposited amorphous silicon carbon (a-SiC:H) solar cells using conventional prior art techniques often experienced greater than 70% degradation in efficiency after a few hundred hours of simulated solar irradiation.
In the past, amorphous silicon and its alloys were deposited by glow discharge with a small amount of hydrogen dilution at a temperature of at least 250.degree. C. and a pressure less than 0.5 Torr. Typifying this conventional glow discharge process and other processes are those described in U.S. Patent Nos. 4,064,521;4,109,271;4,142,195; 4,217,148;4,317,844;4,339,470;4,450,787;4,481,230;4,451,538;4,776;894;and 4,816,082. These conventional prior art processes have met with varying degrees of success. Previously, those skilled in photovoltaic and amorphous silicon deposition arts generally believed that low temperature deposition can produce only inferior products with poor stability and low quality.
It is, therefore, desirable to provide an improved process to produce amorphous silicon devices having improved properties.